1. Field of the Invention
The present invention relates to a keyboard instrument incorporating a sound system for generating musical tones and, more particularly, to an electric or electronic keyboard instrument designed to reduce a sound system in size and improve sound quality or frequency characteristics.
2. Description of the Prior Art
In keyboard instruments, design, frequency characteristics, operability, and the like are very important factors.
In conventional electric or electronic keyboard instruments incorporating speakers, regarding the design, in order to satisfy a demand for a low-profile keyboard instrument, a speaker unit is arranged at a rear portion of a keyboard section. These speaker units are fixed so that their sound radiating directions (axial direction of a diaphragm) are directed to various directions. In a conventional instrument, a slit (tone escape) is formed in the upper surface of an instrument main body case, and the diaphragm of the speaker is arranged to oppose the slit in a substantially horizontal state.
In another electric or electronic keyboard instrument, a speaker is arranged to generate musical sounds toward the rear portion of the main body case. That is, a tone escape is arranged to be open to a side opposite to a performer.
With such a conventional arrangement of a speaker, however, it is difficult for a performer to directly grasp the sound quality of generated tones. Conventionally, the performer grasps performance sounds from sounds reflected by the wall of the rear portion of the instrument main body. In addition, peripheral units such as an MIDI unit cannot be mounted on, e.g., the main body case.
Regarding the frequency characteristics of such a keyboard instrument, for example, an 88-key piano has a lowest bass tone (A.sub.0) of 27.5 Hz, and the frequency of a fundamental wave of a bass drum during automatic rhythm performance is about 30 Hz. These ultra low bass tones pose no problem to the performer in monitoring (grasping) a normal performance, even though a fundamental wave itself is not produced enough. This is because if harmonic waves are reproduced, the bass tones are compensated in audible levels. However, for example, in the bass drum, if a fundamental wave of about 30 Hz is slightly output at a level exceeding an audible sound pressure limit, and a harmonic overtone of 50 to 60 Hz is sufficiently output, the generated tone is felt as a heavy bass tone by the performer. In contrast to this, if a sound system having of lowest reproduction frequency of about 70 Hz or more is used, generated tones become less richer in low frequency region, thus exhibiting a great difference in sound quality.
Many recent keyboard instruments employ a PCM sound source as a sound source. For this reason, if input signals to the sound system are directly reproduced, the sound quality of reproduced tones is very high. In order to reproduce musical tones with high fidelity, a strong demand has arisen for a sound system with improved fidelity. The reproduction characteristics of a sound system are mostly determined by the reproduction characteristics of a speaker system.
A sound system incorporated in conventional keyboard instruments comprises a closed or phase-inversion (bass-reflex) type speaker system and a power amplifier, having a substantially zero output impedance, for constant-voltage driving the speaker system. In this case, the lowest reproduction frequency of the speaker system is mainly determined by the volume of a cabinet (e.g., a main body case) and the characteristics (f.sub.0, Q.sub.0, and the like) of a speaker unit used in the system. That is, in the conventional keyboard instruments, if musical tones having lower frequencies are to be produced, a cabinet having a larger volume is required, resulting in considerable limitation in design. In addition, performance may be interfered depending on an arrangement of the cabinet, and other problems are posed in terms of operation. FIG. 16 shows an outer appearance of a keyboard instrument designed by integrally forming a rear frame 21 and a cabinet 7. Referring to FIG. 16, reference symbols 9a and 9b respectively denote bass-reflex ports (resonance ports).